The present invention relates to a non-reciprocal circuit element such as an isolator or a circulator.
Non-reciprocal circuit elements such as isolators or circulators are used in mobile wireless equipments such as cellular phones. Such a non-reciprocal circuit element is constituted by accommodating magnetic parts and electric parts in a magnetic metal casing serving as a yoke. The magnetic parts include a magnetic rotor constituted by a soft ferrite substrate, a central electrode, etc., a permanent magnet, and so on. The electric parts include a matching capacitor, a terminating resistor, and so on.
The soft ferrite substrate is combined with the central electrode so that a DC magnetic field is applied from the permanent magnet to the soft ferrite substrate. The central electrode includes a plurality of central conductors. One end of the central electrode is disposed on one surface of the soft ferrite substrate so as to be grounded as a ground portion on the metal casing. The central conductors are disposed on the other surface of the soft ferrite substrate so as to be insulated from one another while crossing one another at a predetermined angle. The front end of each central conductor is connected to an electric part such as a matching capacitor or a terminating resistor, and extracted to the outside of the metal casing so as to serve as an external terminal.
The market demands to miniaturize such a non-reciprocal circuit element infinitely. The present dimensions of the non-reciprocal circuit element have been reduced to be 4 mm square or smaller. With this miniaturization, the magnetic rotor or the permanent magnet constituting the non-reciprocal circuit element has been improved to be smaller in size and lower in profile. Thus, those parts have been mounted in high density in the metal casing. In addition, recently, the operating frequency band of such a non-reciprocal circuit element has reached a GHz band. When the non-reciprocal circuit element mounted thus in high density is operated by a high frequency signal, there occurs a problem that a high frequency current is generated in the metal casing.
That is, in the aforementioned non-reciprocal circuit element, a magnetic metal material forming the casing is disposed around the magnetic rotor and the permanent magnet and in close contact. Therefore, when a high frequency signal is supplied to the central conductors constituting the magnetic rotor, a high frequency current is induced in the casing due to an inductance component of the central conductors and an inductance component of the casing. This high frequency current is wasted as Joule loss. On the other hand, the high frequency current orbiting the casing acts to cancel the signal flowing in the central conductors. Thus, the high frequency magnetic field of the magnetic rotor is weakened to degrade the electric characteristic of the non-reciprocal circuit element.
In order to solve such a problem, for example, JP-A-2001-177308 discloses a technique in which ferrite and a gap for blocking an orbiting current (high frequency current) flowing around a central electrode are provided in a metal casing. Further improvement in the characteristic is, however, desired.